For BLACKBERRY (and probably for other models)Open GmailClick menuClick on Accounts (you will be able to see the accounts you have added before)Select the account you want to deleteClick again menuClick again on Accounts (you will be able to see the account which is in use. it has a check/tick mark)Click on EXIT (its the last option)

Try that on the phone please as it doesn't delete - not on my one at any rate - it says Exit Gmail, which it does, and that stops you having new emails forwarded but when you open the app again it will still be there. It has to be "Sign Out".